One benefit of trade is that it:

a. allows for increased specialization and mass-production techniques that lower per unit costs of production.
b. decreases economies of scale, making production more efficient.
c. increases the division of knowledge because trade makes people more self-sufficient, producing more of what they consume.
d. lowers productivity, leading to greater domestic employment.

The answer to this question is A: Allows for increased specialization and mass production techniques that lower per unit costs of production

Explanation:

First, What is Trade?

Trade is the exchange of goods and services. It is the satisfaction of wants, the next step after goods have been produced.

Trade, whether domestic or international requires firm or an economy to exchange goods and services which they have produced for income. A country usually trades goods which it has abundance raw materials  and required resources to produce or focuses on the aspect in which it has comparative advantage.  

If a country therefore focuses on trade in which it has abundance of resources or comparative advantage, it automatically become specialized in the production of such goods and begins to develop more techniques and tools that will ensure that it produce such goods at optimum level that will help it in reducing cost of production for such goods or services.

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